Again I couldn't wait to see the idea coming to life, so I hacked together a quick prototype: I used a stepper motor as a generator, wound a string around its shaft and used a rubber bungie cord to pull the string back after a pull. This string I tied to the apple tree in our garden. Unfortunately today is a day without any wind, so I had to test it by moving the tree manually. As you can see the LED I connected to the stepper motor was lighting up. Yeah! :) Credits for the idea with the string goes to Florian, who posted in the comment section!

Here you can see the tree:

And finally the first energy* harvested! :)

* = Unfortunately only Tobiwans energy, but a update when it will get windy the next time will follow soon!